Characterization of the solidity of knotted single-twine diamond mesh netting

Solidity is an essential parameter of netting in fishing and aquaculture applications, since it is used to derive and apply empirical formulae of drag and lift coefficients. This work derives an analytical formula for calculating the solidity of knotted polyethylene single-twine diamond mesh netting, a very common netting in the fishing industry. It also introduces the concept of line-of-sight solidity at a given angle of attack, as opposed to traditional normal solidity, which could help better explain the hydrodynamic behavior of this kind of netting under different conditions. Based on a detailed 3D geometric model of the netting, an analytical expression for the line-of-sight solidity is also derived, as a function of twine diameter, mesh length, mesh opening, orientation (T0 or T90) and angle of attack of the netting. The analytical expression for solidity was validated with experimental measurements from digital images of netting at different configurations, showing excellent agreement across the entire range of parameters. It was investigated how different netting parameters influence the solidity, which increases non-linearly as the angle of attack decreases. It was found that the normalized solidity, the ratio between line-of-sight solidity and normal solidity, does not depend the twine diameter or mesh length, except in some particular cases. In general, the line-of-sight solidity begins to be greater than the normal solidity for angles of attack less than approximately 65°, although this depends on the orientation and mesh opening of the netting.